(function(doc , win){
var docEl = doc.documentElement,
	resizeEvt = 'orientationchange' in window ? 'orientationchange' : 'resize',
	recalc = function() {
		var clientWidth = docEl.clientWidth;
		if(!clientWidth) return;
		if(clientWidth >= 640) {
			docEl.style.fontSize = '100px';
		} else if(clientWidth <= 320) {
			docEl.style.fontSize = '50px';
		} else {
			docEl.style.fontSize = 100 * (clientWidth / 640) + 'px';
		}
	};

if(!doc.addEventListener) return;
win.addEventListener(resizeEvt, recalc, false);
doc.addEventListener('DOMContentLoaded', recalc, false);
})(document, window);